When a hydrocarbon type heavy oil such as a petroleum heavy oil, coal tar or oil sand is carbonized by heat treatment at 400.degree. to 500.degree. C., microcrystals called mesophase microspheres are formed in the molten heat-treated pitch obtained at the early stage of the heat treatment. The mesophase microspheres are liquid crystals having specific molecular arrangements. They are carbonaceous precursors for affording highly crystalline carbonized products. Also, since they themselves have high chemical and physical activities, they are expected, by being isolated from the above mentioned heat-treated pitch (isolated mesophase microspheres are generally called as mesocarbon microbeads), to be utilized for a wide scope of applications having high added values, including that as starting materials for high-quality carbon materials and starting materials for carbon fibers, binders, adsorbents, etc.
For isolation of such mesophase microspheres, there has been proposed a method in which only the pitch matrix containing these microspheres dispersed therein was dissolved selectively in quinoline, pyridine, or an aromatic oil such as anthracene oil, solvent naphtha, or the like, and the mesophase microspheres as insolubles are recovered by solid-liquid separation. However, in order to perform the heat treatment while avoiding coke formation, the content of the mesophase microspheres in the heat-treated pitch (as determined quantitatively as quinoline insolubles according to Japanese Industrial Standards JIS K2425) can be increased only to at most 15% by weight. It is also necessary to use a solvent in an amount of 30 times or more the weight of the heat-treated pitch. Accordingly, in the method for isolating the mesophase microspheres by selective dissolution of the matrix pitch as described above (hereinafter sometimes referred to as "the solvent separation method"), it is necessary to use a solvent in an amount of 200 times or more the mesophase microspheres to be obtained, whereby productivity is inevitably extremely lowered.
In view of the state of the art as described above, we have previously developed and proposed a process for producing continuously mesocarbon microbeads (isolated product of mesophase microspheres) by means of a liquid cyclone (Japanese Patent Application No. 238/80; U.S. patent application Ser. No. 222,901). This process can enhance productivity by consistent continuity of the steps and effective utilization of solvents and may be considered to be effective as a method for production of mesocarbon microbeads. However, this method, which belongs basically to the solvent separation method, also entails the disadvantage of employing a large quantity of a solvent.